The present invention generally relates to communication systems, and more particularly, to a high-data-rate optical communication system.
The need for high-performance optical communication techniques has become more apparent with the move towards ultra-high-speed 100-Gbps-class all-optical networks. High-sensitivity optical communication links are vital for the design of future high-performance communication networks. For highest sensitivity, it is usual to attempt to match the receiver response to the transmitted waveform. Sensitive receiver performance reduces transmitter or mid-span amplifier requirements, extends link distances, and provides additional margin. This is especially beneficial for free space communications since improvement in receiver sensitivity directly reduces transmitted power requirements.
Communication engineers generally strive to match the receiver response to the transmitted waveform in order to maximize the signal-to-noise ratio (S/N or SNR). This is usually accomplished using a filter which determines the response of the receiver. A filter with a response that is matched to the transmitted waveform is called a matched filter, and maximizes the SNR when the noise is additive white noise. Other methods are known within the art. In radio-frequency systems, an alternative to using a matched filter in the receiver is to use a correlation receiver to maximize SNR; in theory, a correlation receiver can achieve the same SNR as a receiver using a matched filter. The present invention is an optical implementation of a correlation receiver. The SNR also can be increased by providing a higher level of signal output power from the transmitter, if higher power is available.
There has been the desire to achieve the theoretical performance limit. The theoretical performance limit for sensitivity varies according to the system being used, and may be expressed as the energy per bit or the number of photons per bit required to achieve a specified bit error rate (BER). By way of example, the theoretical performance limit for the sensitivity of a Differential Phase Shift Keying (DPSK) system using an optical pre-amplifier may be near 20 photons per bit if the sensitivity is defined for a bit error rate of 1 per billion or 1/1,000,000,000. The theoretical performance limit for an On-Off Keying (OOK) system using an optical pre-amplifier may be 38 photons per bit if the sensitivity is defined for a bit error rate of 1 per billion or 1/1,000,000,000. High-data-rate optical communication systems using On-Off Keying (OOK) or Differential Phase Shift Keying (DPSK) have only recently approached the theoretical performance limit.
It is known within the art to utilize a matched-filter receiver to maximize the SNR. One system, as disclosed by D. O. Caplan & W. A. Atia, “A quantum-limited optically-matched communication link,” Optical Fiber Communication Conference and Exhibit 2001, vol. 1, Mar. 19, 2001, pp. MM2-1-MM2-3, using OOK was able to approach the theoretical limit for an optically preamplified OOK system by employing Return-to-Zero (RZ) OOK modulation and a matched optical filter. Previously, the system described by W. A. Atia & R. S. Bondurant, “Demonstration of return-to-zero signaling in both OOK and DPSK formats to improve receiver sensitivity in an optically preamplified receiver,” LEOS '99 IEEE Lasers and Electro-Optics Society 1999 12th Annual Meeting, vol. 1, 1999, pp. 226-227, demonstrated that the use of return-to-zero (RZ) signaling results in improved receiver sensitivity when compared to the more widely used non-return-to-zero (NRZ) format for the case when non-ideal optical and electrical filters are used. Caplan & Atia were able to improve the receiver sensitivity even more, to approach the theoretical limit for an optically preamplified OOK system, by using a matched optical filter instead of a non-ideal optical filter. However, matched optical filters are custom components and lack ease of modification to match different waveforms. It is desirable to provide a system and method for optical communication that approaches the theoretical performance limit without requiring customized equipment.
D. O. Caplan & W. A. Atia, “A quantum-limited optically-matched communication link,” Optical Fiber Communication Conference and Exhibit 2001, vol. 1, Mar. 19, 2001, pp. MM2-1-MM2-3, disclose a prior-art OOK matched-filter system, depicted in FIG. 1. As shown, there is a transmitter 10 and a receiver 20. The transmitter 10 consists of a laser 12, and a first Mach-Zehnder 14 that is sinusoidally driven to carve out return-to-zero (RZ) pulses. These pulses are subsequently modulated by a LiNbO3 Mach-Zehnder modulator 16 to encode 5 Gb/s non-return to zero (NRZ) data. The Mach-Zehnder modulator 16 is followed by a saturated erbium-doped fiber amplifier 13. The receiver 20 comprises a low-noise erbium-doped fiber amplifier 30 (EDFA) followed by an optional high-speed optical demultiplexer 27, a 0.1 nm Gaussian optical filter 28, a photodetector 29, and an error detector 26. There is also a power meter 25 to measure the power.
FIG. 2, as disclosed by Atia et al., depicts an optically preamplified DPSK system. The transmitter comprises a laser 34, a first modulator 36, and a second modulator 38, wherein the second modulator 38 is a data encoding modulator and is configured as a phase modulator. There is also a variable attenuator 40, a power meter 42, a pulse pattern generator 44, and a clock 46. The receiver 33 incorporates a erbium-doped fiber amplifier (EDFA) 48, a 26 GHz Fabry-Perot optical filter 50, a Mach-Zehnder demodulator with a 1-bit time delay interferometer 52 followed by a 10 GHz balanced detector. The error detector 54 is part of the measurement system which takes the place of the decision circuit or clock-and-data-recovery circuit of a practical system. If the Fabry-Perot optical filter were replaced by a matched optical filter, this system would represent a matched-filter system for DPSK. However, a matched filter may be a custom part which must be fabricated. Also known within the art are radar systems employing radio-frequency correlation receivers in lieu of radio-frequency matched filters. However, optical communications systems require optical implementations, such as the present invention.
As can be seen, there is a need for a high data rate optical communication system that approaches the theoretical performance limit and that may be re-configured for different waveforms without the need for custom optical filters. Also, an alternative means of approaching the theoretical performance limit allows choices which may prove easier to obtain or implement, or less costly.